To apply an aluminum alloy sheet as automobile body panels, it is necessary to use a press die to form it into a desired shape. The 5000-series aluminum alloy sheet controlled in texture and excellent in so-called press-formability has been developed. The 5000-series aluminum alloy sheet is high in strength due to the Mg forming a solid solution in the matrix and further is controlled in texture so is also excellent in press-formability, so has been used in the past as a material for automobile body panels.
For example, in Patent Literature 1, (PLT 1), an Al—Mg-series alloy sheet excellent in deep drawability has been developed. The sheet disclosed in PLT 1 contains 2 wt %-6 wt % Mg, and one or more of the elements Fe, Mn, Cr, Zr, and Cu totaling at least 0.03 wt % (when Cu is selected, the Cu comprises at least 0.2 wt %). Compositional upper limits disclosed are 0.2 wt % Fe, 0.6 wt % Mn, 0.3 wt % Cr, 0.3 wt % Zr, and 1.0% Cu, with a balance of Al and unavoidable impurities. The sheet in PLT 1 additionally has a texture with a ratio of CUBE orientation volume fraction to S orientation volume fraction (S/Cube) of 1 or more, at most 5 vol % GOSS orientation, and has a grain size of 20 to 100 micro-meters.
Furthermore, automobile body panel is coated and baked after press-forming, so sheet excellent in so-called “bake hardness” has been sought. For this reason, 6000-series aluminum alloy sheet controlled in texture and excellent in so-called press-formability has also been developed.
For example, Patent Literature 2 (PLT 2) further described regarding the texture of an aluminum alloy or an aluminum alloy sheet (below, “aluminum alloy sheet”), an aluminum alloy sheet for press-forming use characterized in that the orientation density of the CR orientation ({001}<520>, same below) is higher than the orientation density of all other orientations besides the CR orientation. The sheet disclosed in PLT 2 contains Si: 0.2% to 2.0% (mass %, same below) and Mg: 0.2% to 1.5%, further contains one or more of Cu: 1.0% or less, Zn: 0.5% or less, Fe: 0.5% or less, Mn: 0.3% or less, Cr: 0.3% or less, V: 0.2% or less, Zr: 0.15% or less, Ti: 0.1% or less, and B: 0.005% or less, and has a balance of unavoidable impurities and aluminum. According to this, by setting the rolling direction of cold rolling with respect to the rolling direction of hot rolling to become 90 degree, it is possible to raise the breaking limit at equal biaxial deformation, plane strain deformation, and monoaxial deformation and provide aluminum alloy sheet suitable for press-forming.
Furthermore, Patent Literature 3 (PLT 3) describes a high formability Al—Mg—Si-series alloy sheet treated with solution treatment. The sheet disclosed in PLT 3 contains Mg: 0.3 to 2.0% (mass %, same below) and Si: 0.3 to 2.5%, has a balance of Al and unavoidable impurities. The sheet has a structure in which the area ratio of the grains having {432} planes inclined within 9.0 degree in range from parallel with the sheet surface to the total area of the grains of all crystal orientations is 0.15 or more, alpha/beta is 2.0 or more when the highest among the orientation distribution functions of the orientations comprised of {111}<112>, {332}<113>, {221}<114>, and {221}<122> is a alpha and the higher among the orientation distribution functions of the orientations comprised of {001}<100> and {001}<110> is beta, and the average Lankford value is 0.9 or more.
In this regard, an automobile body panel requires hemming so as to crimp an outer panel and inner panel together. However, the 6000-series aluminum alloy sheet is inferior to 5000-series aluminum alloy sheet in so-called bendability etc., so fine cracks and surface roughness after bending have to be prevented. Furthermore, while thinner gauge and higher strength are demanded, springback at the time of press-forming has to be suppressed. In particular, with bending, there are many cases where defects such as fine cracks which seems to be caused by formation of high density shear bands occur. Suitable control of the recrystallized texture has become an issue.
For example, in PLT 1, an Al—Mg—Si alloy sheet material was used as a test material to prepare a single crystal. The effect which each crystal orientation has on bendability was studied in detail from the viewpoint of the formation of shear bands. According to this study, it became clear that there is a close relationship between the crystal orientation and bendability. In the study, the bendability of the test piece having the <001>//ND orientation was the best. Further, the bending anisotropy was also the smallest.
Furthermore, Patent Literature 4 (PLT 4) describes an aluminum alloy sheet excellent in formability. The sheet disclosed in PLT 4 contains Fe: 1.0 to 2.0 mass % and, further, Mn: 2.0 mass % or less, having a balance of aluminum and unavoidable impurities, and restricted in Ti as an unavoidable impurity to 0.01 mass % or less. The sheet has a structure with an average grain size of 20 micro-meters or less and an area rate of {110} oriented crystal of 25% or more. According to this, by electromagnetically stirring while DC casting, it is possible to achieve all of an elongation of 35% or more, an average r-value of 0.85 or more, a ball head bulging height of 33 mm or more, and a limited drawing ratio of 2.17 or more.